The invention relates generally to the commercial and industrial refrigeration art, and more particularly to a multiple compressor oil system for commercial and industrial refrigeration.
The maintenance of a proper amount of lubricating oil in the compressor of any refrigeration system obviously is a critical factor to the efficient operation and life span of the compressor. Oil problems are particularly acute in large multiplexed or compounded systems in which multiple compressors operate in parallel or series-piped arrangements and pump into a common discharge header to provide the refrigeration needs of commercial installations, such as supermarkets which have a large number of low and/or normal temperature refrigerated display and storage fixtures, or for industrial installations, such as warehousing having a plurality of different refrigeration requirements.
In all operating refrigeration systems, some amount of oil is entrained in the hot compressed refrigerant vapor discharged by the compressors and generally some oil is present throughout the entire system, including condenser, receiver, evaporator coils, liquid and suction lines, valves, etc. It is clear that compressor lubricating oil serves no useful purpose outside the compressor, that energy is wasted by pushing oil through the refrigeration system, that oil interfers with the heat transfer and efficiency of evaporators and that oil may create system damage due to oil build-up interferring with proper refrigerant distribution, valve operation and the like. Therefore, high side oil traps or separators have been employed between the compressor and condenser to separate the oil from the refrigerant that is passed on to the condenser and thus minimize such oil distribution through the system. It is desired to return the oil in liquid form to the compressors and various high side and low side oil devices have been used, such as sumps, accumulators, pumps, oil float controls, valves and the like.
Refrigerants such as R-12, R-22 and R-502 are miscible with the lubricating oil, and generally some amount of refrigerant will be present in any oil separation system. However, in prior oil separator systems, the cooling of separated oil below the condensing temperature of the gas refrigerant frequently produced excessive refrigerant condensation in and dilution of the oil. Such oil and refrigerant solution results in reduction of lubrication quality and excessive pump-out of the oil into the system. Excessive oil foaming also occurred in some cases of crankcase pressure reduction such as during compressor start-up following a long off-cycle. In addition to problems of inefficient oil-refrigerant separation, a major problem has been the maintenance of proper oil levels between multiple and cyclically operating compressors. A typical solution in the past was to return the oil to the suction header for the compressors and allow the oil to flow into the warm refrigerant vapor and at random into the compressors without regard to different pumping rates, and then attempt to provide an oil level equalizing connection between the compressor crankcases, such as is disclosed in U.S. Pat. No. 3,140,041. U.S. Pat. No. 3,633,377 also discusses a high side oil separator, accumulator and muffler for a multiple compressor system that approaches some of the oil problems.
While numerous oil separation devices and systems have been developed in the past, efficient oil separation and maintenance of proper oil levels in multiple compressor systems has continued to present oil problems in refrigeration systems.